How to scale the texture used by Displace modifier
Displacement map made with procedural textures

Switch to Textures panel in order to open texture used by Displace modifier. You will find different options which vary depending on texture type. The Size option is accessible for many of them which is one you need if you work with procedural textures.



Image textures

If you have image texture, I'd suggest changing texture coordinates which use Displace modifier. 


In modifier's panel, Texture Coordinates option, change it to Object. 
Add a simple plane somewhere nearby, choose it as object in mentioned option.  
Now to scale image which is used as displacement map scale that added mesh. Displacement map will be resized according to that: